1. Field of the Invention
The present invention relates to a hydraulic system and more particularly to a method of controlling the wicket gates of a pump-turbine incorporated in the hydraulic system.
Generally, various components such as runner of a pump-turbine, particularly of a high-head type, are designed to achieve sufficient centrifugal pump action so as to obtain the high-head during pump operation. However, this design badly affects the turbine operation of the pump-turbine. When the performance of the pump-turbine designed in this manner is shown by the performance curve representing the relationship between speed per unit head N.sub.1 and discharge per unit head Q.sub.1 under a predetermined opening degree of wicket gates, this curve includes in the turbine operation area a first section where the value of Q.sub.1 reduces with the increase of the value of N.sub.1 and a second section where the value of Q.sub.1 reduces with the decrease of the value of N.sub.1. In this specification, the second section will be referred to as "S-section" for convenience of explanation. Further, the pump-turbine performance in the S-section will be hereinafter referred to as "S-performance." During the turbine operation in the S-section, the value of torque per unit head T.sub.1 also is reduced as the value of speed per unit head N.sub. 1 decreases, while in the first section value of T.sub.1 reduces as the value of N.sub.1 increases.
Ordinarily, the turbine operation of the pump-turbine is effected in the abovementioned first section. However, in the case where the speed per unit head N.sub.1 is suddenly increased because of for example a removal or loss of load carried on the pump-turbine, the pump-turbine initiates to operate in the S-section. When the operation in the S-section is initiated, the discharge per unit head Q.sub.1 and speed per unit head N.sub.1 are first reduced tracing the S-section from one end to the other, and thereafter Q.sub.1 and N.sub.1 are increased tracing the S-section in the opposite direction. This reciprocal tracing on the S-section is repeated endlessly and would never be terminated without taking particular measures. Also the torque per unit head T.sub.1 is repeatedly reduced and increased during this pump-turbine operation. The pump-turbine operation in the S-section causes in the penstock and tailrace as well as in the pump-turbine a disadvantageous abnormal hydraulic pressure variation inclusive of large pressure rise and drop with the resultant severe water hammer and, in an extreme case, water column separation. It is to be noted that the abovementioned removal of load would occur, if for example the pump-turbine were driving a generator which lost its load due to the failure or burning out of a transformer, and that the water hammer is very severe when the penstock or tailrace, or both, is long.
There is the case where a plurality of turbine operation means such as turbines and/or pump-turbines are incorporated in the hydraulic system, and a plurality of hydraulic pipelines each extending across each of the turbine operation means are joined at their upstream ends to an upstream common pipeline connected to an upper reservoir and/or at their downstream ends to a downstream common pipeline connected to a lower reservoir. In this case, the hydraulic system is designed to comprise a plurality of turbine operation means, penstock means including a plurality of pipelines each extending from the upstream side of each of the turbine operation means and communicated with the upper reservoir, and tailrace means including a plurality of pipelines each extending from the downstream side of each of the turbine operation means and communicated with the lower reservoir, at least one of the upstream ends of the pipelines of the penstock means and the downstream ends of the pipelines of the tailrace means being connected through a multi-branched manifold to a common pipeline which in turn is connected to the corresponding reservoir. When the hydraulic system is designed in this manner, the operation of one of the turbine operation means affects the operation of the other turbine operation means. If the plurality of turbine operation means do not include the pump-turbine having the abovementioned S-performance, the most severe condition causing the maximum hydraulic pressure in the penstock means as well as the minimum hydraulic pressure in the tailrace means occurs when the plurality of turbine operation means have lost their loads simultaneously. It is here assumed that the plurality of turbine operation means are arranged substantially symmetrically such that the pipelines of the penstock means and/or pipelines of the tailrace means which are connected to the common pipeline have substantially the same lengths. It will be understood that, when the plurality of turbine operation means do not include the pump-turbine having the S-performance, the entire hydraulic system can be designed under consideration of the most severe condition described above.
If however the plurality of turbine operation means include one or a plurality of pump-turbine or pump-turbines having the S-performance, there is a possibility that the disadvantageous pressure variation caused during the pump-turbine operation in the S-section is accelerated due to the operation of the other turbine operation means. The pump-turbine operation thus affected by the other turbine operation means further affects the operation of the latter turbine operation means. The disadvantageous pressure variation is unexpectedly uncontrollably multiplied in this manner. If such multiplication effect occurs, the simultaneous removal of loads from the turbine operation means does not always cause the most severe condition. For example, when the plurality of turbine operation means have lost their loads successively with certain time lags, the hydraulic pressure rise in the penstock means and the hydraulic pressure drop in the tailrace means may become extremely severe because of the multiplication effect described above. It cannot be easily analyzed under what operational conditions of the respective turbine operation means the most severe condition occurs, and there are cases where the hydraulic pressure variation caused under the most severe condition is extremely larger than the pressure variation caused when the plurality of turbine operation means have lost their loads simultaneously. Further, the extremely larger pressure drop caused in the tailrace means under the most severe condition occasionally causes the water column separation in the tailrace means. If the entire hydraulic system were designed to withstand such larger pressure variation, it would become extremely uneconomical.
As will be understood from the foregoing, when the turbine operation means include the pump-turbine or pump-turbines having the S-performance, and the values of the maximum pressure in the penstock means and the minimum pressure in the tailrace means are determined as being subjected to the multiplication effect during the pump-turbine operation in the S-section, it is difficult to control such maximum and minimum pressures.
2. Description of the Prior Arts
There have been proposed various wicket gate controlling methods intending to reduce the disadvantageous hydraulic pressure variation by controlling the closure of the wicket gates upon sudden removal of load from the pump-turbine. However, these methods cannot prevent the abovementioned multiplied hydraulic pressure variation caused while the pump-turbine is being operated in the S-section as being affected by the operation of the other pump-turbine or turbine. It is considered that the prior art methods have been proposed without sufficiently analyzing and understanding the S-performance of the pump-turbine.